The invention relates to a process for roller spot welding of cylindrically shaped container bodies, particularly food cans in a material comprising a mild steel sheet having on at least one of its faces a protective conductive coating whose contact resistivity with respect to copper is at least 1.times.10.sup.-5 ohm/cm.sup.2.
More particularly the invention relates to a roller spot welding process in which each surface of the sheet is coated with a layer of tin containing at most 1.05 g/m.sup.2 covered with a composite layer of chromium and chromium oxide consisting of 5 to 140 mg/m.sup.2 of metallic chromium and 1 to 30 mg/m.sup.2 of chromium in oxidised form.
In roller spot welding, overlapping edges of a body are rolled together along a longitudinal welding line while the body advances at a speed which can reach at least 50 m/min., and is held between two resistance welding electrodes.
French Pat. No. 1,258,185 describes a welding machine in which the two electrodes are rollers, and a copper wire is interposed between the rollers and the faces of the body along the longitudinal welding line. The diameter of the roller which is applied to the inner face of the body is less than that of the body so that the body can pass over that roller, while the other roller has a greater diameter.
Similar machines are known, in which the electrode over which the container body passes is a block, which is rectilinear or has a large radius of curvature, and which is mounted facing the external roller electrode.
Roller spot welding is a particular type of resistance welding, in which the passage of current between two electrodes which are applied to overlapping sheets causes localised melting of the metal of the sheets where they are in contact, with cross penetration of the molten metal. A good quality spot weld requires that metal of both sheets form the molten zone, without that zone extending through to the external surfaces of the sheets. The heating which occurs is proportional to the contact resistances between faces of the sheets and corresponding electrodes, the resistance of the thickness of the sheets, and the contact resistance between the lapped sheets.
In addition because the electrodes are relatively massive and are made of a metal which is a good conductor of heat, they drain thermal energy from the spot weld and, in particular cool the areas which they are in contact. Electrical and thermal conductivities of metals are related, both being due to electron movements.
The intrinsic electrical resistance of the sheets is well defined, even if the current flow is not everywhere normal to the welding plane. The contact resistance between different metals is however quite different from a theoretical ohmic resistance, and depends on a number of conduction mechanisms which are generally non-linear. A contact resistance cannot be defined without specifying the measurement conditions, essentially current density and contact pressure; and in order to obtain significant measurements it is desirable to specify operating conditions close to those which are encountered during welding.
In French Pat. No. 2,465,011 (corresponding to U.S. Pat. No. 4,421,828 --Alloue and Mergey), the Applicants have described a method of measurement according to which the coated sheet whose contact resistivity is to be measured is held between two copper electrodes each having a contact area of 10 mm.sup.2, with a clamping force of 1,400 Newtons; a current of 1 ampere is passed between the electrodes, and the voltage drop between the electrodes is measured. This voltage is between 50 microvolts and a few millivolts for the usual materials.
This method of measurement is comprehensive, in that it takes into account the contact resistivity between each electrode and the coated face to which it is applied. From this there has been developed a method of deriving the contact resistivity at the interface of an electrode and a coated face. It was found that the contact resistivity between the copper electrodes themselves was about 50 microohms, and that the overall contact resistivity of tin plate did not differ significantly from this value. Additionally values of substantially the same order were found with any coated steel sheet, by removing the coatings by abrasion, and by electrolytically tinning the exposed surfaces to produce a coating corresponding to a typical tin plate (at least 1.2 g/m.sup.2 and preferably 2.8 g/m.sup.2 per face). The intrinsic resistance of the mild steel sheet, taken transversely on a section of 10 mm.sup.2, is about 2.5 microohms. As a result, the contact resistivity between copper and the coating of one face can be determined by locally removing the coating of the other face, electrolytically coating with tin the area from which the coating has been removed, and measuring the contact resistivity by the method previously defined, with an electrode in contact with the tin coating.
The text which follows will refer to contact resistivity relative to copper, in terms of resistance per unit surface area. By convention, the square centimeter will be used as the unit of surface area, and the values will be expressed in 10.sup.-5 ohm/cm.sup.2. If the unit is not explicitly specified, it is this unit, 10.sup.-5 ohm/cm.sup.2, which is used.
It will be noted that the contact resistance between the faces of sheets which are applied one against the other along the longitudinal welding line causes heating at the exact place where the temperature must be at a maximum, whilst the contact resistances between faces of sheets and electrodes are located where the heating must be a minimum. A priori, it would therefore be desirable that the contact resistance between the contacting faces of sheets be at a maximum, and the contact resistances between the faces of the sheets and the electrodes which contact them be at a minimum. But a correlation exists between the contact resistances between sheets, and between electrodes and sheets. In addition, there is the influence of the thickness of the sheets which affects the heating in the mass of the sheets, and the rapidity of diffusion of the heat in the welding area towards the electrodes.
All these parameters which determine the quality of the resistance welding are supplemented by parameters intrinsic to roller spot welding, particularly to roller spot welding of cylindrical container bodies under the conditions mentioned above.
A first constraint is due to the speed of passage of the bodies. A spot weld is formed at each half cycle of an alternating voltage at a frequency of a few hundred Hertz, which is applied between the electrodes by an appropriate transformer. Regulation of the energy dissipated in the spot welds is obtained by regulation of the conduction angle of controlled discharge semi-conductors connected in the primary circuit of the transformer. Even when the conduction angle is carefully stabilised, problematical variations in the electrical properties of the sheets and of the electrodes give rise to fluctuations in the quality of the spot welds. The weldability of sheets, such as those which constitute the bodies of food cans, not only means their suitability for spot welding after a suitable adjustment. The probability must also be considered of the appearance of defective spot welds during production with a usual setting, as a consequence of problematical variations of the properties of the sheets and of the electrodes. This aspect of the quality of the welding can be expressed, at least qualitatively, by a range of variation of the conduction angle within which acceptable results in production are obtained. This range will be called welding latitude.
Other constraints particular to the roller spot welding of cylindrical bodies will be mentioned later.
French Pat. No. 2,465,011 referred to above relates to a material having layers of uncombined tin on the two faces of a mild steel sheet, covered with a composite layer of chromium and chromium oxide containing from 50 to 100 mg/m.sup.2 of metallic chromium and from 6 to 25 mg/m.sup.2 of chromium in oxidised form. European Patent Application No. EP 25,396 specifies that the layer of tin is from 0.1 to 1.5 g/m.sup.2.
This material is noteworthy for its weldability by roller spot welding, in that it has not been heated, prior to welding, so that a negligible part of the tin is alloyed with the underlying iron. This excludes lacquering of the sheet with a polymerised organic lacquer prior to welding, at a temperature above or equal to 200.degree. C.
Japanese Patent Application No. 56-119796 (1981) describes a chrome steel strip with, on one face, 5 to 50 mg/m.sup.2 of metallic chromium and 2 to 30 mg/m.sup.2 of chromium in oxidised form, and on the other face, 0.5 to 10 mg/m.sup.2 of metallic chromium and 2 to 30 mg/m.sup.2 of chromium in oxidised form. This material may be roller spot welded, at a speed of 20 m/min. The weldability is due to the fact that the total quantity of metallic chromium is limited, which correspondingly reduces the overall electrode/sheet/sheet/electrode contact resistance. In addition, a coating comprising from 0.5 to 10 mg/m.sup.2 of metallic chromium ensures a suitable protection for the external surface of a can body, which is to be coated with a film of paint, the role of the metallic chrome being to ensure the bond between the steel and the chromium oxide. The work of the Applicant company however leaves doubts as to the pertinence of the mechanism referred to in this Japanese application, without, however, casting doubts on the weldability of the material at the specified speed of passage and the production of the welding quality referred to.
French Patent Application No. 81 21,384 (FR-A-2,516,553) decribes a steel sheet which is coated on one side with a layer of tin between 0.2 and 10 g/m.sup.2, and on the other side with a composite layer of metallic chromium (20-250 mg/.sup.2) and of chromium in an oxidised form (5-40 mg/m.sup.2). This material can be roller spot welded at an unspecified speed of passage. The weldability is attributed to the fact that the contact between the two faces of the sheet only comprises one thickness of chromium and one thickness of chromium oxide instead of two as for prior materials, and this is stated to reduce the resistance of the current path. Moreover, the tin is stated to protect against one kind of corrosion and the composite layer against another kind of corrosion. For example, the tin is stated to be suitable for contact with food products, and the composite layer of chromium and chromium oxide is stated to be effective against corrosion due to atmospheric agents.
The conclusions drawn by the Applicants from their work on weldability by roller spot welding do not agree with the views expressed in No. FR-A-2,516,553, on the weldability of the material described in that prior application. The preferred arrangement in No. FR-A-2,516,553 is tin on the internal face of the body of the can, which is not particularly effective for internal lacquering, since tin is not a very effective bonding surface for lacquer, particularly in the case of thin coatings (0.2 to 1.05 g/m.sup.2). As already stated chromium/chromium oxide coatings have the effect, amongst others, of improving the bonding of lacquers. This would lead in many cases to using thicknesses of tin sufficient in themselves to protect the sheet against corrosion, that is to say using quantities of tin comparable to those of the conventional tin plate. Conventional tin plate can, of course, be roller spot welded without any great difficulty. Further the Applicants' experience leads them to believe that an installation capable of reliably depositing tin on one face of a sheet, and a composite chromium/chromium oxide coating on the other face, would be complex and be subject to production problems.